1. Field of the Invention
The present invention provides compositions and methods for preventing and treating viral infections. The present invention thus has applications in the areas of medicine, pharmacology, virology, and medicinal chemistry.
2. The Related Art
Few good options are available for preventing or treating viral infections. The vast majority of antiviral drugs interfere with viral replication through the inhibition of transcription of the viral genome. Commonly these drugs inhibit a specific protein involved in viral genomic transcription, such as a polymerase or transcriptase; which often produces unwanted toxicity, since viruses depend largely on host factors for viral genomic replication. Moreover, given the highly specific nature of the target, small mutations in the viral genome are often sufficient to create viral strains that are resistant to chemotherapeutics. In addition, since the drugs inhibit active viral replication, they cannot eliminate virus that is latent or sequestered in the host; thus, patients are forced to take antivirals and endure their toxic effects for long periods if not indefinitely. Not surprisingly, patients on such regimens cannot continue treatment, and remain infected as well as providing a potentially continuing source of additional infections.
Thus there is a need for better antiviral chemotherapeutics and more effective strategies for identifying such chemotherapeutics. The need is especially urgent for those suffering from chronic and debilitating viral infections, such as human immunodeficiency virus (HIV) and hepatitis C(HCV), for which no good treatment exists for the reasons noted above.
But new viral threats are also on the horizon. The steady encroachment of civilization into the most remote regions of the globe has introduced the risk of exotic viral infections to the population at large. Each passing year brings an increasing number of reports of infections by hemorragic fevers, such as Ebola virus (EBOV), Marburg virus (Marburg), and Rift Valley Fever virus (RVFV). Still other viral infections can cause potentially debilitating effects, such as recurrent fevers, joint pain, and fatigue; these include: Punta Toro Virus (PTV), West Nile virus (WNV), chikungunya virus (CHK), Easter Equine Encephalitis virus (EEEV), Western Equine Encephalitis virus (WEEV), Lhasa virus (LASV), and Dengue virus (DENV).
By way of example, one of the additional “new” viruses (that is, new with respect to the industrialized world) is Venezuelan Equine Encephalitis virus (also called Venezuelan equine encephalomyelitis, VEEV). VEEV is a mosquito-borne viral disease of all equine species, including horses, asses (wild and domestic), and zebras. Equines infected with VEEV may show one or more of the following signs: fever, depression, loss of appetite weakness, and central nervous system disorders (lack of coordination, chewing movements, head pressing, “sawhorse” stance, circling, paddling motion of the limbs, and convulsions). In some cases, horses infected with VEEV may show no clinical signs before dying. The clinical signs of VEEV can be confused with those of other diseases that affect the central nervous system. These include eastern equine encephalitis, western equine encephalitis, African horse sickness, rabies, tetanus, and bacterial meningitis. VEE might also be mistaken for toxic poisoning. Definitive diagnosis can be made by isolating the virus in a laboratory or by testing blood for the presence of antibodies to the virus.
Humans also can contract this disease. Healthy adults who become infected by the virus may experience flu-like symptoms, such as high fevers and aches; and those having weakened immune systems, as well as the young and elderly, can become more severely ill or even die.
The virus that causes VEEV is transmitted primarily by mosquitoes that bite an infected animal and then bite and feed on another animal or human. The speed with which the disease spreads depends on the subtype of the VEEV virus and the density of mosquito populations. Enzootic subtypes of VEEV are diseases endemic to certain areas. Generally these serotypes do not spread to other localities. Enzootic subtypes are associated with the rodent-mosquito transmission cycle. These forms of the virus can cause human illness but generally do not affect equine health. Epizootic subtypes, on the other hand, can spread rapidly through large populations. These forms of the virus are highly pathogenic to equines and can also affect human health. Equines, rather than rodents, are the primary animal species that carry and spread the disease. Infected equines develop an enormous quantity of virus in their circulatory system. When a blood-feeding insect feeds on such animals, it picks up this virus and transmits it to other animals or humans. Although other animals, such as cattle, swine, and dogs, can become infected, they generally do not show signs of the disease or contribute to its spread.
Naturally occurring outbreaks of VEEV are rare. In 1936, VEEV was first recognized as a disease of concern in Venezuela following a major outbreak of equine encephalomyelitis. From 1936 to 1968, equines in several South American countries suffered devastating outbreaks. In 1969, the disease moved north throughout Central America, finally reaching Mexico and Texas in 1971. The highly pathogenic form of VEEV has not occurred in the United States since 1971. However, in 1993 an outbreak of VEEV in the State of Chiapas, Mexico, prompted the U.S. Department of Agriculture to temporarily increase its surveillance activities and tighten its quarantine requirements for equine species entering the United States from Mexico. During outbreaks, the most effective way to prevent further spread of disease is to quarantine infected equines. Controlling mosquito populations through pesticide treatments and eliminating insect-breeding sites will also enhance disease control. These measures should be accompanied by a large-scale equine immunization program. Equines in the United States should be vaccinated for VEE only when there is a serious threat that the disease could spread to this country.
Similar to VEE is West Nile virus (WNV), which was mentioned above. West Nile virus is named for a district in Uganda where the virus was first identified in humans in 1937. Outbreaks of the virus have occurred in a number of countries throughout Europe, the Middle East, Africa, Central Asia, and Australia, since that time. WNV was first detected in the Western Hemisphere in 1999, and since then the disease has spread across North America, Mexico, Puerto Rico, the Dominican Republic, Jamaica, Guadeloupe, and El Salvador. Symptoms range from a mild, flu-like illness (fever, headache, muscle and joint pain) and a red, bumpy rash, to meningitis. In rare cases those infected will develop encephalitis, which can include high fever, a stiff neck, disorientation, paralysis, convulsions, coma, and death in about ten percent of cases.
No cure or treatment is available for either VEEV or WNV, or the other viruses listed above; so public health experts emphasize prevention by avoiding areas where the disease has been detected or where disease vectors (usually mosquitoes) have been identified. However, that approach is becoming less reasonable as the world population grows. Moreover, some officials fear that one or both of these diseases, or other similar viruses in the toga- and flaviviridae, could be “weaponized” by a hostile government or terrorist organization to immobilize military personnel or important segments of the population in an attack.
To make matters still more complicated, the above-mentioned viral threats span almost all of the recognized viral families, including the bunyaviruses, flaviviruses, filoviruses, arenaviruses, and togaviruses. Since viral families are defined in significant part by their differences in mechanism for genomic replication, therapeutic strategies that are focused on inhibiting genomic replication will be inadequate for large outbreaks of new, and especially weaponized, viruses.
PCT Publication WO 2008/124550 discloses small molecule therapeutics having “broad spectrum” antiviral properties. Nevertheless, there remains an acute need to provide medicinal treatments for viral diseases. The present invention meets these and other needs.